1. Field of the Invention
This invention relates to the field of control and actuation of automatic transmissions for automotive vehicles. More particularly, the invention pertains to a system for controlling synchronous engagement and disengagement of friction elements that control operation of a multiple-speed transmission.
2. Description of the Prior Art
When gear ratio changes are produced in an automatic transmission, one friction element, a clutch, brake or brake band, is disengaged and a second friction element changes from the disengaged state to the engaged state. The speed ratio produced by the transmission is changed, and the torque transmitted by the various friction elements changes during the gear ratio change. To avoid harsh gear ratio changes during synchronous gear shifts various techniques are used to apply torque slowly to the oncoming friction element and to coordinate its engagement with disengagement of the offgoing friction element.
Attempts have been made to use electric timers to assure that the state of the oncoming and offgoing friction elements are coordinated in time and do not occur concurrently. However, the use of timers has produced unsatisfactory results because of the effect of temperature variations, manufacturing tolerances and differences from car line-to-car line, even where the same powertrain is employed.
In an electronically controlled automatic transmission operating in overdrive, a downshift from the overdrive speed ratio to an direct drive speed ratio occurs when the vehicle operator depresses an overdrive cancel switch, thereby producing an electronic signal but without effecting a change of position of any component of the hydraulic control circuit. Formerly such gear ratio changes required the operator to move a gearshift lever from the overdrive position to the direct drive position, thereby changing the position of a manual valve in the hydraulic control system. The downshift produces enhanced engine braking.
These problems are particularly important in automatic transmission wherein an overdrive brake band must release its engagement of an overdrive brake drum before a forward clutch is applied in order to produce a 4-3 downshift. If the forward clutch is applied before the overdrive drum is released, the output shafts of the transmission are locked to the transmission case. This action stops rotation of the drive wheels, an entirely unacceptable outcome. Furthermore, if the forward clutch is applied long after the overdrive drum is released, there will result a period during which there is no available engine braking. This latter condition is entirely unacceptable also because a desire for increased engine braking that causes the vehicle operator to downshift from the overdrive speed ratio.